U.S. Patent Application Publication 2002/0186214A1, by Siwinski, published Dec. 12, 2002, shows a method for saving power in an organic light emitting diode (OLED) display having pixels comprised of red, green, blue and white light emitting elements. The white light emitting elements are more efficient than the other colored light emitting elements and are employed to reduce the power requirements of the display. In a power saving mode, only the white emitting elements are employed to display a black and white image that uses less power. The white emitting elements are not used in the full color display mode.
While power efficiency is always desirable, it is particularly desirable in portable applications because an inefficient display limits the time the device can be used before the power source is recharged. In fact, for certain applications the rate of power consumption may be more important than any other display characteristic with the exception of visibility.
It is also known to provide an OLED display having pixels with differently sized red, green and blue light emitting elements, wherein the relative sizes of the elements in a pixel are selected to extend the service life of the display. See U.S. Pat. No. 6,366,025 B1, issued Apr. 2, 2002 to Yamada.
It has been known for many years that the human eye is most sensitive to green light and less sensitive to red and blue light. More specifically, the spatial resolution of the human visual system is driven primarily by the luminance rather than the chrominance of a signal. Since green light provides the preponderance of luminance information in typical viewing environments, the spatial resolution of the visual system during normal daylight viewing conditions is highest for green light, lower for red light, and even lower for blue light when viewing images generated by a typical color balanced image capture and display system. This fact has been used in a variety of ways to optimize the frequency response of imaging systems. For example, as described in U.S. Patent Application Publication 2002/0024618 A1, by Imai, published Feb. 28, 2002, in a pixel having a square array of red, green, blue and white light emitting elements, the colors green and white having large luminance components are positioned diagonally opposite. However, this design does not provide increased power efficiency for an emissive full-color display.
There is a need, therefore, for an improved OLED display having improved power efficiency, resolution, and extended lifetime.